Combustion of gaseous fuel

ABSTRACT

A fuel gas is passed into one set of channels in a compact reactor ( 12 ) consisting of a plurality of metal sheets ( 41 ) arranged to define first and second gas flow channels ( 14  and  15 ), the channels being arranged alternately to ensure good thermal contact between the gases in them and each channel containing a removable metallic heat conducting insert ( 44 ) coated with a ceramic. In the set of channels carrying the fuel the ceramic supports particles of a transition metal oxide, which is reduced by the combustion gas to form metal particles. In the other set of channels the ceramic supports particles of a transition metal, and these channels carry a flow of an oxidizing gas, which oxidises the metal. The flows to the two sets of channels are then exchanged. If the oxidizing gas is steam, the result is a stream of pure hydrogen.

The invention relates to an apparatus and a method suitable for performing combustion of gaseous fuel, and which may be used for producing hydrogen.

Fuel cells consuming hydrogen and oxygen offer the promise of providing clean power for motor vehicles. However, this leads to a requirement for an efficient and correspondingly clean process for the production of hydrogen. Steam reforming is a common method of hydrogen production. The main process step involves the reaction of steam with a hydrocarbon over a catalyst to form hydrogen and carbon oxides. However, the subsequent process steps that are needed to separate the hydrogen from the carbon oxides and any impurities are complicated and expensive. Likewise, it is difficult to separate out hydrogen from the combustion gases that are formed upon the air gasification of a fuel such as a fossil based hydrocarbon or solid biomass. The present invention enables pure hydrogen to be produced, while overcoming these problems.

It is recognised that the release of carbon dioxide into the atmosphere as a result of the combustion of fossil fuels, such as coal, methane or petrol may, in the long-term, have detrimental effects on the Earth's climate because increasing concentrations of carbon dioxide in the atmosphere may contribute to global warming. It has therefore been suggested that sequestration of carbon dioxide gas would be desirable. However carbon dioxide in exhaust gases is typically present in a dilute form, because air is used in the combustion process. One possible solution is to scrub carbon dioxide from the flue gas, so that the resulting concentrated carbon dioxide can be injected into a suitable storage medium such as saline aquifers, geologic formations, or in depleted hydrocarbon reservoirs. An alternative solution would be to use pure oxygen as the combustion gas. Both these solutions are expensive, and the present invention enables this problem to be overcome.

According to the present invention there is provided an apparatus for performing combustion of a gaseous fuel, the apparatus comprising a compact reactor consisting of a plurality of metal sheets arranged to define first and second gas flow channels, the channels being arranged alternately to ensure good thermal contact between the gases in them and each channel containing a removable metallic heat conducting insert coated with a support ceramic, in one set of channels the ceramic incorporating particles comprising an oxide of a transition metal and in the other set of channels the ceramic incorporating particles of a transition metal.

In use of the apparatus, an oxidizing gas is passed through the channels containing the particles of the transition metal, while a gaseous fuel is passed through the channels containing the particles of the oxide.

The oxide or metal particles are preferably less than 50 μm in size, more preferably less than 20 μm, and may be considerably smaller, down to a size of about 10 nm. The transition metal is preferably one or more selected from chromium, copper, cobalt, nickel, iron or manganese, and the proportion of the transition metal is preferably in the range 5-50%, more preferably 5-40% by weight of the support ceramic. The metal or metal oxide may be introduced by impregnating the ceramic with a solution of a salt of the metal, followed by drying and thermal decomposition in either a reducing or an oxidising environment respectively. Appropriate catalyst materials such as ruthenium, palladium or platinum may also be provided on the insert in each channel, which can catalyse both the oxidation and reduction reactions.

Suitable materials for the ceramic are those which are stable at the reaction temperatures and do not react irreversibly with the transition metal, for example alumina or zirconia. The ceramic may also be doped with a material such as lanthanum, cerium or gadolinium to enhance its stability.

It should be appreciated that the “metal” particles might actually be a metal oxide in which the metal is in a low oxidation state, whereas in the “metal oxide” particles the metal is in a higher oxidation state.

To ensure the required good thermal contact, both the first and the second gas flow channels are preferably less than 8 mm deep in the direction normal to the sheets. More preferably both the first and the second gas flow channels are less than 5 mm deep, but preferably at least 0.5 mm deep. The heat-conducting insert may comprise a corrugated or dimpled foil, a wire mesh, or a corrugated metal felt. The ceramic coating is of thickness typically in the range between 30 and 300 μm, and is porous, the transition metal or metal oxide particles being dispersed within the porous ceramic, and the ceramic being sufficiently porous that the gaseous reagents can diffuse to the surface of the particles. The specific surface area of the ceramic is preferably in the range 50-340 m²/g, and the ceramic may be, for example, lanthanum-stabilised gamma-alumina.

It will be appreciated that the materials of which the reactor are made are subjected to a severely corrosive atmosphere in use, for example the temperature may be as high as 900° C., although more typically around 850° C. The reactor may be made of a metal such as an aluminium-bearing ferritic steel, in particular of the type known as Fecralloy (trade mark) which is iron with up to 20% chromium, 0.5-12% aluminium, and 0.1-3% yttrium. For example it might comprise iron with 15% chromium, 4% aluminium, and 0.3% yttrium. When this metal is heated in air it forms an adherent oxide coating of alumina which protects the alloy against further oxidation; this oxide layer also protects the alloy against corrosion under conditions that prevail within the reactor. Where this metal is coated with a ceramic layer, the alumina oxide layer on the metal is believed to bind with the ceramic coating, so ensuring the ceramic adheres to the metal substrate under conditions of thermal cycling. An alternative metal would be Inconel (trade mark) 800HT.

Accordingly, the present invention also provides a method for performing combustion of a gaseous fuel, the method using a compact reactor consisting of a plurality of metal sheets arranged to define first and second gas flow channels, the channels being arranged alternately to ensure good thermal contact between the gases in them and each channel containing a removable metallic heat conducting insert coated with a support ceramic, in the first flow channels the ceramic incorporating particles comprising an oxide of a transition metal and in the second flow channels the ceramic incorporating particles of a transition metal, and the method comprising two steps carried out alternately:

a) supplying a gaseous fuel to the first flow channels containing oxide particles, and supplying an oxidizing gas to the second flow channels containing metal particles;

b) supplying the said gaseous fuel to the second flow channels and supplying the oxidizing gas to the first flow channels;

each step being carried out for sufficient time that a substantial proportion of the oxide particles have been reduced, and the gas flows then being exchanged so the other step is carried out.

The gaseous fuel comprises at least one gas which reduces the transition metal oxide particles to metal particles. It may comprise more than one reductant and may also include a diluent such as nitrogen and or carbon dioxide. If the combustion gas does not contain a diluent such as nitrogen, the output from the combustion channels can be substantially pure CO₂ (once any water-vapour has been condensed).

The oxidizing gas may comprise oxygen, for example it may be air. Alternatively the oxidizing gas may comprise an oxygen-containing compound, such as water vapour. If the oxidizing gas is steam, then the method provides an output of hydrogen gas, which can be over 99% pure.

The gaseous fuel may be the result of gasifying a hydrocarbon or a biomass product. For example, biomass such as forestry waste, coppiced willow or rice/corn husk may be decomposed autothermally (partial oxidation), typically in a fluidized bed gasifier, so as to produce hydrogen, carbon monoxide, methane, water, nitrogen and ash. For example, a typical gas composition for a 20% moisture wood feed is 50-54% N₂, 17-22% CO, 9-15% CO₂, 12-20% H₂ and 2-3% CH₄. Such a gas has a typical heating value of 5-5.9 MJ/Nm³. The fluidized bed gasifier may be fed with pre-heated air so that partial oxidation of the biomass occurs which produces the heat for the remaining thermal decomposition of the biomass. The gasifier may in addition be fed with steam.

The gas resulting from the gasification of the biomass is typically cleaned, for example using catalytic tar removal, cyclonic ash removal and filtration. The gas is then fed into the channels of the reactor. The reductants in the gas, such as carbon monoxide, hydrogen and methane, react with the transition metal oxide to produce carbon dioxide and water, and the transition metal oxide is reduced to the metal. The reaction is exothermic, and is effectively combustion of the gas. Typically the reaction proceeds at about 300-800° C.

This process enables an impure mixture of gases to be used to generate hydrogen indirectly, by reducing the metal oxide to the metal. The metal oxide is re-formed by the reaction between hot steam and the metal, which results in the generation of pure hydrogen without the need for complicated separation techniques.

Alternatively, the first set of channels may be fed with a gaseous fuel which undergoes combustion in the channels, but which does not include any diluent gases. Any gaseous fuel that is capable of reducing the metal oxide may be used, for example synthesis gas produced from coal or heavy oil gasification. The fuel is oxidized in the channels, thereby reducing the metal oxide to metal. The metal oxide acts as an oxygen donor for the reaction thus producing the fully oxidized pure combustion products carbon dioxide and water. The carbon dioxide is not diluted with nitrogen because the source of oxygen for the combustion is not air but a metal oxide. The pure carbon dioxide can then be sequestrated by compression and injection into a suitable sub-surface storage volume without the need to remove any nitrogen.

If the oxidizing gas is steam, this may be produced by heating water. The steam reacts with the dispersed transition metal particles at elevated temperature, typically 300-800° C., to produce transition metal oxide and hydrogen. The reaction is endothermic and heat for the reaction is provided from the exothermic reaction of the gaseous fuel with the transition metal oxide in the adjacent set of channels.

If the oxidizing gas is air, the oxygen from the air reacts exothermally to generate the metal oxide. As a result the temperature may reach above 800° C., and heat is transferred into adjacent channels. In the channels carrying the fuel, the fuel reacts with the metal oxide, and is itself oxidized. The absence of a flame front and the good thermal control result in low NO_(x) generation. The overall chemical reaction is that the gaseous fuel is oxidized, so the overall process is strongly exothermic. The reactor may incorporate a third set of channels which in this case may carry a coolant fluid.

Once the reduction and/or the oxidation reaction is substantially completed, the gas streams feeding each set of channels are changed over so that the reactions proceed in alternating quasi-continuous cycles. The gas flows are exchanged when a substantial proportion of the metal oxide particles have been reduced, and this proportion is preferably at least 30%, more preferably 50%, more preferably 70% and most preferably 90% of the particles.

In one embodiment, the hot product gases, hydrogen, carbon dioxide and water vapour from the reactor are used to pre-heat the air that is fed to the fluidised bed gasifier. For example, the hot hydrogen product and/or the hot exhaust gas from the transition metal oxide reduction reaction may be used in this way.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings in which:

FIG. 1 shows a flow diagram for the plant and process of the invention;

FIG. 2 shows a sectional view of a compact reactor for use in the plant of FIG. 1, showing one metal plate in plan; and

FIG. 3 shows a flow diagram for an alternative plant and process of the invention.

Referring to FIG. 1, a plant 10 for combustion of a fuel gas is shown, where the fuel gas may be for example methane, or a mixture of hydrogen and carbon monoxide resulting from the gasification of coal or other fossil fuels. The plant 10 incorporates a compact reactor 12 which defines two sets of gas flow channels 14 and 15 which alternate with each other and are in good thermal contact (the reactor is described in greater detail with reference to FIG. 2). Within each flow channel 14 and 15 is a removable metallic heat-conducting insert coated with a ceramic; initially the inserts in flow channels 14 contain particles of cobalt metal, while the inserts in flow channels 15 contain particles of cobalt oxide. The reactor 12 incorporates headers 18 in communication with the flow channels 14, and incorporates headers 19 in communication with the flow channels 15.

In the first phase of operation the fuel gas is supplied through a valve 20 to the inlet header 19 to flow through the channels 15, and at the same time air is supplied through a valve 22 to the inlet headers 18 to flow through the flow channels 14. Oxygen from the air reacts vigorously and exothermically with the particles of cobalt metal, oxidising them to cobalt oxide, this reaction increasing the temperature in the channels 14 to about 800° C. Because of the good heat transfer through the inserts and between adjacent channels 14 and 15, the inserts in the channels 15 are also heated to a high temperature, typically about 750° C. At this high temperature the fuel gas reduces the cobalt oxide particles to cobalt metal, this reaction being somewhat exothermic; the fuel gas undergoes combustion, by taking oxygen from the oxide particles.

The hot gases emerging from the outlet headers 18 and 19 are consequently at about 750° C., and are then supplied via respective valves 24 and 25 to respective heat exchangers 26 to generate steam. This may be used for electricity generation. The exhaust gases from the channels 15, in which the fuel gas has been oxidised, are then cooled still further to condense water vapour, as indicated at 28, and the resulting carbon dioxide is subjected to sequestration treatment.

Once substantially all the metal oxide particles have been reduced to metal (in the channels 15) or substantially all the metal particles have been oxidised to metal oxide (in the channels 14), the gas flows are exchanged by operating the valves 20 and 22 and the valves 24 and 25. Thus in the second phase the fuel gas flows through the channels 14, and the air flows through the channels 15. These two phases then alternate whenever substantially all the metal oxide particles in the channel carrying the fuel gas have been reduced to metal. Hence combustion of the fuel gas is substantially continuous, though taking place alternately in the two sets of channels. The cycle time for switching between the two phases depends on the operating temperature, the metal loading and the degree of metal dispersion.

It will be appreciated that there is very good thermal contact between the oxidation and reduction reactions, so that the processes are thermally integrated, and the development of hot or cold spots is suppressed. The good thermal transfer means that the reactor 12 can be comparatively small.

Referring now to FIG. 2, a reactor 40 suitable for use as the reactor 12 comprises a stack of Fecralloy steel plates 41, each plate being generally rectangular, 450 mm long and 150 mm wide and 3 mm thick, these dimensions being given only by way of example. There may for example be forty plates 41 in the stack. On the upper surface of each such plate 41 are rectangular grooves 42 of depth 2 mm separated by lands 43 (eight such grooves being shown), but there are three different arrangements of the grooves 42. In the plate 41 shown in the drawing the grooves 42 extend diagonally at an angle of 45° to the longitudinal axis of the plate 41, from top left to bottom right as shown. In a second type of plate 41 the grooves 42 a (as indicated by broken lines) follow a mirror image pattern, extending diagonally at 45° from bottom left to top right as shown. In a third type of plate 41 the grooves 42 b (as indicated by chain dotted lines) extend parallel to the longitudinal axis.

The plates 41 are assembled in a stack, plates 41 of the third type (with the longitudinal grooves 42 b) alternating with plates of the first or second types, which are themselves used alternately so that each plate of the third type is between a plate with diagonal grooves 42 and a plate with mirror image diagonal grooves 42 a, and after assembling many plates 41 the stack is completed with a blank rectangular plate. The plates 41 are compressed together during diffusion bonding, so they are sealed to each other. Inserts 44 comprising corrugated Fecralloy alloy foils (only one is shown) 50 μm thick coated with a ceramic coating, of appropriate shapes and with corrugations 2 mm high, can be slid into each such groove 42, 42 a and 42 b.

Header chambers 18 are welded to the stack along each side, each header 18 defining three compartments by virtue of two fins 47 that are also welded to the stack. The fins 47 are one third of the way along the length of the stack from each end, and coincide with a land 43 (or a portion of the plates with no groove) in each plate 41 with diagonal grooves 42 or 42 a. Gas flow headers 19 in the form of rectangular caps are then welded onto the stack at each end, communicating with the longitudinal grooves 41 b. (In a modification (not shown), in place of each three-compartment header 18 there might instead be three adjacent header chambers, each being a rectangular cap like the headers 19.) Hence the longitudinal grooves 42 b define the channels 15, while the diagonal grooves 42 and 42 a define the channels 14.

In use of the reactor 40, the flow path for the mixture supplied to the top-left compartment of the header 18 (as shown) is through the diagonal grooves 42 into the bottom-middle header compartment, and then to flow through the diagonal grooves 42 a in other plates in the stack into the top-right compartment of the header 18. Hence the gas flows in the channels 15 and 14 are at least partially co-current.

The headers 18 and 19 each comprise a simple rectangular cap sealed around its periphery to the outside of the stack so as to cover part of one face of the stack. They may be welded onto the outside of the stack. Alternatively, if neither of the gas flows are at elevated pressures, it may be adequate to clamp them onto the outside of the stack. In either case it will be appreciated that after a period of use, if the metal or metal oxide particles in either or both of the channels has lost reactivity, then the headers 18 and 19 may be removed or cut off and the corresponding inserts 44 removed and replaced. The headers 18, 19 can then be re-attached.

The inserts 44 may have a coating of stabilised gamma alumina of thickness 100 μm, to which cobalt metal is added in the proportion 30% by weight. After deposition of the ceramic coating, the coating is saturated with a solution of cobalt nitrate, and this is then decomposed by heat treatment. For those inserts 44 that are required to contain cobalt oxide, the heat treatment can be carried out in an atmosphere containing oxygen, whereas for the inserts 44 that are required to contain cobalt metal the heat treatment would be carried out in a reducing atmosphere, for example containing hydrogen. This process may be repeated to increase the proportion of cobalt. This process can produce particles of size less than 50 nm, which are highly reactive. The alumina support maintains the high degree of dispersion of the cobalt (or cobalt oxide) preventing sintering during high temperature operation; it may incorporate additives such as lanthanum to improve hydrothermal stability.

It will be appreciated that the reactor 12 may take a different form to that shown in FIG. 2, but that it is essential that the channels 14 and 15 are in good thermal contact with each other. Hence the reactor should define a multiplicity of alternating flow channels, and these are preferably defined between metal sheets in a stack, either by flat sheets with grooves or slots, or by corrugated sheets. Preferably the channels are not in transverse directions, so the flows are at least partly parallel, but the headers for the different gases must be separate. Furthermore the inserts must be removable, once the headers are removed, so they can be replaced if necessary without replacing the entire reactor.

In one alternative the reactor may also incorporate a third set of flow channels for generating steam. For example the reactor may comprise a stack of hexagonal plates each defining a set of straight grooves between a pair of opposite sides; plates arranged with their grooves in different orientations provide flow paths for different fluids, so there are flow paths for three different fluids: water/steam; fuel (to reduce metal oxide particles); and air (to oxidize metal particles). These three fluid flow paths may be defined by three successive plates, such groups of three successive plates being repeated to form the stack. This enables superheated steam to be generated directly from the reactor. The reactor may operate at a lower temperature to that discussed above, for example 400-600° C., or even as low as about 300° C. if the metal is highly dispersed and a reduction promoter such as ruthenium and platinum is included in the ceramic.

Referring to FIG. 3, a plant 30 for the production of hydrogen is shown. The plant 30 incorporates a compact reactor 12 which defines two sets of gas flow channels 14 and 15 which alternate with each other and are in good thermal contact, and the reactor 12 may have the structure described above. Within each flow channel 14 and 15 is a removable metallic heat-conducting insert coated with a ceramic; initially the inserts in flow channels 15 contain particles of cobalt metal, while the inserts in flow channels 14 contain particles of cobalt oxide.

In the first phase of operation, a fuel gas, resulting in this case from the gasification of biomass, is supplied through a valve 31 to flow through the channels 14, and at the same time steam is supplied through a valve 32 to flow through the flow channels 15. The steam reacts endothermically with the particles of cobalt metal, oxidising them to cobalt oxide.

The fuel gas in this case is made from a supply of biomass 33. The biomass 33 is fed to a gasifier 34 where it undergoes partial oxidation in the presence of air. The resulting fuel gas is then cleaned and sulphur is removed 35 from it. The gas is fed through a heat exchanger 36 so as to heat water to produce the steam for channels 15, and then fed through the valve 31 to the channels 14. The hot fuel gas reduces the cobalt oxide particles to cobalt metal, this reaction being somewhat exothermic. Thermal balance in the reactor may be achieved by the control of reactant gas flowrate within each set of channels 14 and 15.

The hot gases emerging from the channels 14 and 15 are typically at about 750° C., and are then supplied to heat exchangers 26. The heat exchangers are positioned so as to provide some or all of the heat required to heat the air that is fed to the fuel gasifier 34. The exhaust gases from the channels 14, in which the fuel gas has been oxidised, may then be cooled still further to condense water vapour, if this water is required in the steam generating process, and the remaining gases are vented to the atmosphere. One or both of the heat exchangers 26 may alternatively be positioned so as to heat the steam for the compact reactor.

Once substantially all the metal oxide particles have been reduced to metal (in the channels 14) and substantially all the metal particles have been oxidised to metal oxide (in the channels 15), the gas flows are exchanged by operating the valves 31 and 32 and valves 37 and 38 in the outlet ducts. Thus in the second phase the fuel gas flows through the channels 15, and the steam flows through the channels 14. These two operational phases then alternate whenever substantially all the metal particles in the channel carrying the steam have been oxidised to metal oxide. Hence, hydrogen production is substantially continuous, though taking place alternately in the two sets of channels. The cycle time for switching between the two phases depends on the operating temperature, the metal loading and the degree of metal dispersion and the space velocity of the feed gases. So as to maintain a substantially pure flow of hydrogen the exhaust gases from the channels fed by steam may initially be vented to the atmosphere after changeover until sufficiently pure hydrogen is being produced.

It will again be appreciated that there is very good thermal contact between the oxidation and reduction reactions, so that the processes are thermally integrated. The use of highly dispersed metal within a porous ceramic carrier means that it is very active, and that mass transport of reaction and product gases to and from the metal surface is far better than in the case of the use of bulk metal. The combination of good heat and mass transfer improves the overall kinetics of the reactions leading to high volumetric productivity of hydrogen product per unit volume of reactor.

It will be appreciated that as an alternative, a compact reactor may be supplied with a fuel gas with no diluent gases (as described in relation to the plant 10), fed to the channels containing metal oxide particles, and with steam (as described in relation to the plant 30) fed to the channels containing metal particles. In this case not only is pure hydrogen gas generated (as with the plant 30), but the resulting CO₂ from the combustion of the fuel gas can readily be sequestered (as with the plant 10). 

1. An apparatus for performing combustion of a gaseous fuel, the apparatus comprising a compact reactor consisting of a plurality of metal sheets arranged to define first and second gas flow channels, the channels being arranged alternately to ensure good thermal contact between the gases in them and each channel containing a removable metallic heat conducting insert coated with a support ceramic, in one set of channels the ceramic incorporating particles comprising an oxide of a transition metal and in the other set of channels the ceramic incorporating particles of a transition metal, and the apparatus comprising means to supply a gaseous fuel to the channels containing the oxide particles and an oxidizing gas to the channels containing the metal particles, and means to exchange the gases supplied to the channels at intervals.
 2. An apparatus as claimed in claim 1 wherein the oxide or metal particles are less than 50 μm in size.
 3. An apparatus as claimed in claim 1 wherein the oxide or metal particles are larger than about 10 nm.
 4. An apparatus as claimed in claim 1 wherein the transition metal is one or more selected from chromium, copper, cobalt, nickel, iron or manganese.
 5. An apparatus as claimed in claim 4 wherein the proportion of the transition metal is in the range 5-50% by weight of the ceramic.
 6. An apparatus as claimed in claim 1 wherein both the first and the second gas flow channels are between 1 mm and 5 mm deep in the direction normal to the sheets.
 7. An apparatus as claimed in claim 1 also comprising sheets to define channels for generating superheated steam.
 8. A method for performing combustion of a gaseous fuel, the method using a compact reactor consisting of a plurality of metal sheets arranged to define first and second gas flow channels, the channels being arranged alternately to ensure good thermal contact between the gases in them and each channel containing a removable metallic heat conducting insert coated with a support ceramic, in the first flow channels the ceramic initially incorporating particles comprising an oxide of a transition metal and in the second flow channels the ceramic initially incorporating particles of a transition metal, and the method comprising two steps carried out alternately: a) supplying a gaseous fuel to the first flow channels containing oxide particles, and supplying an oxidizing gas to the second flow channels containing metal particles; b) supplying the said gaseous fuel to the second flow channels and supplying the oxidizing gas to the first flow channels; each step being carried out for sufficient time that a substantial proportion of the oxide particles have been reduced, and the gas flows then being exchanged so the other step is carried out.
 9. A method as claimed in claim 8 wherein the exhaust gases from the channels to which the gaseous fuel is supplied re processed so as to sequester carbon dioxide.
 10. A method as claimed in claim 8 wherein the oxidizing gas is steam, so as to produce hydrogen as a product. 